[PATCH] KVM: MMU: Never free a shadow page actively serving as a root

We always need cr3 to point to something valid, so if we detect that we're
freeing a root page, simply push it back to the top of the active list.

Signed-off-by: Avi Kivity <avi@qumranet.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 19 insertions, 2 deletions

diff --git a/drivers/kvm/kvm.h b/drivers/kvm/kvm.h
index 201b2735ca91..b24a86e1f434 100644
--- a/drivers/kvm/kvm.h
+++ b/drivers/kvm/kvm.h
@@ -134,6 +134,7 @@ struct kvm_mmu_page {
                                     */
         int global;              /* Set if all ptes in this page are global */
         int multimapped;         /* More than one parent_pte? */
+        int root_count;          /* Currently serving as active root */
         union {
                 u64 *parent_pte;               /* !multimapped */
                 struct hlist_head parent_ptes; /* multimapped, kvm_pte_chain */
diff --git a/drivers/kvm/mmu.c b/drivers/kvm/mmu.c
index 0e44aca9eee7..f16321498093 100644
--- a/drivers/kvm/mmu.c
+++ b/drivers/kvm/mmu.c
@@ -550,8 +550,13 @@ static void kvm_mmu_zap_page(struct kvm_vcpu *vcpu,
                 *parent_pte = 0;
         }
         kvm_mmu_page_unlink_children(vcpu, page);
-        hlist_del(&page->hash_link);
-        kvm_mmu_free_page(vcpu, page->page_hpa);
+        if (!page->root_count) {
+                hlist_del(&page->hash_link);
+                kvm_mmu_free_page(vcpu, page->page_hpa);
+        } else {
+                list_del(&page->link);
+                list_add(&page->link, &vcpu->kvm->active_mmu_pages);
+        }
 }
 
 static int kvm_mmu_unprotect_page(struct kvm_vcpu *vcpu, gfn_t gfn)
@@ -667,12 +672,15 @@ static int nonpaging_map(struct kvm_vcpu *vcpu, gva_t v, hpa_t p)
 static void mmu_free_roots(struct kvm_vcpu *vcpu)
 {
         int i;
+        struct kvm_mmu_page *page;
 
 #ifdef CONFIG_X86_64
         if (vcpu->mmu.shadow_root_level == PT64_ROOT_LEVEL) {
                 hpa_t root = vcpu->mmu.root_hpa;
 
                 ASSERT(VALID_PAGE(root));
+                page = page_header(root);
+                --page->root_count;
                 vcpu->mmu.root_hpa = INVALID_PAGE;
                 return;
         }
@@ -682,6 +690,8 @@ static void mmu_free_roots(struct kvm_vcpu *vcpu)
 
                 ASSERT(VALID_PAGE(root));
                 root &= PT64_BASE_ADDR_MASK;
+                page = page_header(root);
+                --page->root_count;
                 vcpu->mmu.pae_root[i] = INVALID_PAGE;
         }
         vcpu->mmu.root_hpa = INVALID_PAGE;
@@ -691,6 +701,8 @@ static void mmu_alloc_roots(struct kvm_vcpu *vcpu)
 {
         int i;
         gfn_t root_gfn;
+        struct kvm_mmu_page *page;
+
         root_gfn = vcpu->cr3 >> PAGE_SHIFT;
 
 #ifdef CONFIG_X86_64
@@ -700,6 +712,8 @@ static void mmu_alloc_roots(struct kvm_vcpu *vcpu)
                 ASSERT(!VALID_PAGE(root));
                 root = kvm_mmu_get_page(vcpu, root_gfn, 0,
                                         PT64_ROOT_LEVEL, 0, NULL)->page_hpa;
+                page = page_header(root);
+                ++page->root_count;
                 vcpu->mmu.root_hpa = root;
                 return;
         }
@@ -715,6 +729,8 @@ static void mmu_alloc_roots(struct kvm_vcpu *vcpu)
                 root = kvm_mmu_get_page(vcpu, root_gfn, i << 30,
                                         PT32_ROOT_LEVEL, !is_paging(vcpu),
                                         NULL)->page_hpa;
+                page = page_header(root);
+                ++page->root_count;
                 vcpu->mmu.pae_root[i] = root | PT_PRESENT_MASK;
         }
         vcpu->mmu.root_hpa = __pa(vcpu->mmu.pae_root);